It is well established that vitamin D restricts intracellular Mycobacterium tuberculosis (Mtb) replication via induction of the anti-microbial peptide, cathelicidin. However, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), the active form of vitamin D, has a number of immunomodulatory effects on the immune system and these effects favor protection from autoimmune tissue destruction. There is thus a huge interest in the immunotherapeutic potential of vitamin D and its analogs in treating autoimmune and inflammatory conditions. Despite a significant body of evidence showing vitamin D's negative effect on Mtb growth in vitro, the impact of the aggregate effect of vitamin D's anti-microbial activity and opposing immunomodulatory functions on host resistance against Mtb remained undetermined. We therefore examined the effect of 1,25(OH)2D3 treatment on the outcome of Mtb infection in the mouse model. We found that 1,25(OH)2D3-treated animals had increased bacterial burden and this was associated with significantly smaller B cell rich lymphocytic clusters in the lungs. Together with the recent recognition that B cells contribute to the development of the granulomatous response and in maintaining optimal anti-Mtb immunity, our in vivo findings raise a clinically important question of whether vitamin D supplementation in humans will compromise long-term immunity to Mtb. In Aim 1, we will test the hypothesis that the immunomodulatory functions of vitamin D interfere with the establishment in the lungs of the appropriate cytokine/chemokine axis necessary to recruit and maintain B cells for generating optimal granulomatous response and long term anti-Mtb immunity. The murine cathelicidin gene does not contain a vitamin D response element in its promoter. To fully capture the effects of vitamin D on immune cells that would be relevant in humans, we will use a humanized mouse developed by Dr. Adrian Gombart that carries a genomic copy of the human cathelicidin gene. Immunopathology causes mortality in TB patients and it seems likely that IFN? production in the setting of concomitant Mtb disease may contribute to pathogenesis. Unlike the widely used C57BL/6 and BALB/c strains of mice, C3HeB/FeJ mice develop necrotic and hypoxic tuberculosis lesions; pathological features that are akin to human TB. Interestingly, anti-TB drug effects and activity were found to be different in the C3HeB/FeJ mice. Therefore, this mouse model is more appropriate to study effect of adjunct therapies on the efficacy of anti-Mtb drug regimens. In Aim 2, we will test the hypothesis that in Mtb-infected C3HeB/FeJ mice, 1,25(OH)2D3 treatment will dampen inflammation and reduce necrotic lesion development in the lungs. These changes to lung pathology induced by 1,25(OH)2D3 treatment will improve efficacy of anti-TB drug regimens. The multi-PI application brings together the extensive expertise of Christakos and Salgame in vitamin D and mycobacterial immunity respectively, to successfully address the aims of the proposal.